The printing ink used in stencil printing devices in most cases consists of W/O type emulsion ink having an appropriate consistency. Such a printing ink is typically produced in a factory by mixing and emulsifying oil and water containing dye (pigment) by using a mixer, and is commercially available in ink bottles. Since the effective part of the printing ink is limited to the oil component of such an emulsified ink, an bottle of printing ink tends to be consumed by printing a limited number of prints, and it is necessary to replace the ink bottle more often than desired.
It is known that emulsion printing ink tends to undergo some changes in its property and to separate into oil and water in time. Printing ink which has separated into oil and water and is thus made uneven in its properties loses its original consistency (viscosity), and may lose its suitability as printing ink for stencil printing. Therefore, there is a certain service life in the emulsion printing ink packaged in an ink container.
Furthermore, most printing inks for stencil printing such as emulsion inks and oil inks are subjected to changes in their consistency depending on the surrounding temperature, and it is necessary to adjust the contents of the printing ink depending on the season of the year for keeping the consistency or viscosity at an appropriate level irrespective of the surrounding temperature.